[MYNEWT] Allow initialization of HASH when enabled
[MYNEWT] Allow initialization of HASH when enabled

The Mynewt HASH HW driver can be used to do the sha256 validation in the
bootloader instead of relying on mbedTLS/tinycrypt. When enabled it must
be initialized before boot_go (and boot_img_validate) is run, so this
does the extra checking and runs initialization when required.

Signed-off-by: Fabio Utzig <66676b1ceaf93296e098708c09494361103aa635@apache.org>

utzig/mcuboot,ATmobica/mcuboot,runtimeco/mcuboot,utzig/mcuboot,runtimeco/mcuboot,utzig/mcuboot,tamban01/mcuboot,tamban01/mcuboot,ATmobica/mcuboot,runtimeco/mcuboot,ATmobica/mcuboot,tamban01/mcuboot,tamban01/mcuboot,runtimeco/mcuboot,utzig/mcuboot,runtimeco/mcuboot,tamban01/mcuboot,ATmobica/mcuboot,ATmobica/mcuboot,utzig/mcuboot